Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
The conventional windings in transformer coils for the most common high voltage transformers (132 kv: 11 kV or similar) use a disk type winding. In this winding, the wound cable consists of a number of insulated conductors (up to 32, but usually 4 to 8). The conductors are transposed so that each presents the same impedance to the line to ensure a uniform current distribution among the conductors within the cable.
A disk type winding is employed so that the voltage is across the-coil is distributed evenly from top to bottom and that no two disks experience a greater voltage stress than any other disk under normal operating conditions. This type of winding is shown schematically in FIG. 1. In this type of winding the turns 5 of the winding lie on top of each other to form an axially extending disk 6 rather than cylinder formed in a typical solenoid type winding. As shown, the top disk 7 of the coil is at a higher voltage, and the bottom of the disk will be at zero voltage in a star-type connected three-phase transformer, or at the line voltage in a delta-type connected transformer. The disk windings are connected without the need for joints or brazing of any kind, at one end in an alternating manner down the height of the disk stack.
The primary coil (high voltage side) is usually wound in this manner, and the secondary coil is usually wound in a single layer solenoid type winding, because its voltage is less (11 kV), and so does not suffer the same electric stresses. However, if a second layer is required, it cannot be placed on top of the first and is usually placed a sufficient distance away with appropriate press board spacers and oil ducts sufficient to meet the inter-layer voltage stress and cooling requirements.
Solenoid type windings typically consist of a conductor wound so that the turns of the conductor lie side by side to form a helical layer (usually cylindrical), once a layer is completed, further layers may be wound over the first layer in a reciprocal manner until the desired number of layers is formed.
Transformers are not designed to only meet their normal operating voltage stresses. The clearances are decided so that the transformer can withstand the voltage stresses at the prescribed testing conditions set out in the local standards. For example, in the Australian standard, the clearances and barriers must be designed to withstand approximately twice the voltage stress at the power frequency (the so called power frequency or AC test) and an appropriate lightning impulse test. For a 132 kV transformer, the peak of the lightning impulse test will be 550 kV or 650 kV as specified by the customer.
For the above reasons, it is not practical to wind the high voltage primary coil as a solenoid because the voltage stress between the layers at the top and bottom of the coils is high, and increases in proportion with the number of layers times the number of turns per layer. The worst case normal voltage stress will be between the first turn and the last turn at the top of the winding which will have the full 132 kV normal voltage across the annular thickness of the coil (defined as the difference in the outside radius and inside radius of the coil) and under test conditions, significantly more depending on the test. Hence, disk windings are used. A solenoid-type winding is shown in FIG. 2 where the conductor 10 is wound to and fro vertically to form a number of vertically extending layers 11.
In a disk winding the greatest voltage stress during any test is between the top two disks, but these only have a fraction of the total turns in them, so the voltage will only be of the order of kilovolts or tens of kilovolts during L1, not the full 230 (AC test) or 550 kV (L1).
Disk windings require significant handling and physical contortions of the disks and individual copper conductors in order to be wound in a single length in a neat and tidy manner suitable for transformer coils. For example, each second disk must be turned inside out after winding (so that the inner turn becomes the outer and outer one becomes the inner). This is to facilitate winding in a continuous manner, not to ensure transpositioning. In addition, the conductors must be kinked in plane parallel to the width of the conductor in order to go from one disk to the other.
The abovementioned disadvantages are of more concern in the field of High Temperature Superconductors (HTS). It is most likely that HTS transformers will only replace those very large transformers where the savings in weight and size justify the cooling overhead. Hence, the discussion is limited to those cases where the primary voltage is at 110 kV or greater (132, 230, 350, 500 kV for example), however, it will be appreciated the invention is also applicable to other high voltage windings.
HTS conductors cannot be subjected to the level of mechanical manipulation which copper conductors are subjected to during the winding of transformer coils. The unit length of HTS may also not be available in more than 1000 m lengths which means a completely continuous HTS winding is not possible.
One way to wind a high voltage HITS primary coil and avoid the above-mentioned manipulations is to use a series of electrically connected double pancakes. A pancake is analogous to a disk type winding. The double pancakes could be then connected in series with normal conductors or other HTS conductors with resistive joints. Double pancakes must be used instead of single pancakes to avoid connections that run down the side of the plane of each pancake. Double pancakes allow connections to be all on the outside or inside of the stack avoiding cross leads traversing down the radial length of the coils which will pick up flux.
The disadvantage of this type of arrangement, however, is that in large transformers, the number of connections could be quiet large and approach hundreds. The connections are sources of dissipation which add to the losses, and can be sources of bubbling in liquid nitrogen, or thermal instabilities in other types of cooled methods due to the concentrated nature of the loss.
Another way to wind a high voltage coil is to use a continuous solenoid type winding. However, as stated above, this results in a high voltage stress across a short creep distance at the top of the coil. This is further compounded by the fact that HTS windings have significantly less annular thickness and so the electrical stress is increased many fold.
The voltage between layers within the winding is of not much concern because the interlayer insulation is a solid dielectric, preferably Kapton™, which has a high breakdown strength (>80<90 kV/mm at 77K) sufficient to meet the test requirements. However, at the top end of the coil, the layer to layer voltage stress can breakdown along a creepage path 20 through the liquid nitrogen or gaseous nitrogen where no solid dielectric exists. Gaseous nitrogen at 77 K has a corona on-set point of just 5 kV for distances of between 10 and 30 mm and all liquid nitrogen cooled coils will have a gaseous portion above the liquid.
The addition of a solid inter-layer dielectric which extends beyond the top of each layer is possible, however, the liquid nitrogen would still have significant stress because it has the lower relative dielectric constant (1.4 at 77K compared to 3.0 to 3.6 for Kapton™).
The interlayer dielectric would have to be very thick or very long to the extent where the complete winding becomes very large.
Manufacturers typically take significant and costly precautions to pass the lightning impulse test. The additional benefit of a solenoid type winding is that the L1 pulse is distributed more uniformly between the layers within each solenoid and so can prevent the requirement for a shielding winding or an interleaved winding. In a disk winding, without shielding windings or interleaved windings, the L1 produces a very large stress between the top two disks and would fail without the countermeasures in place. However, these countermeasures are expensive to wind, and complicate the winding considerably.